Nothing immoral about selling at market prices, but it might just be illegal to go into the ammunition business without a license, which is what you're describing, and that would be immoral. I'm not talking about people who are selling what they've bought over the years and have decided they have more than they need at today's prices. I'm talking about those who have gone into business illegally to take advantage of a crisis. I would have sympathy for someone needing the cash to scrape by, but I suspect more than a few ammo flippers aren't.
You don't need any kind of license to sell commercially loaded ammunition. You need a Type 6 FFL if you're going to manufacture ammo for sale (reload for profit).
The only way to get into any sort of trouble "flipping" commercial ammo would be if the IRS thought you weren't paying the taxes on the profits.
Lots of small town convenience and hardware stores have a small section of ammo on the shelf sans any manner of FFL related to the business.
Razorback, are you perhaps the President or Sergeant at Arms of your HOA?
